The invention relates to electronic devices, and, more particularly, to speech coding, transmission, storage, and synthesis circuitry and methods.
The performance of digital speech systems using low bits rates has become increasingly important with current and foreseeable digital communications. One digital speech method, linear predictive coding (LPC), uses a parametric model to mimic human speech. In this approach only the parameters of the speech model are transmitted across the communication channel (or stored), and a synthesizer regenerates the speech with the same perceptual characteristics as the input speech waveform. Periodic updating of the model parameters requires fewer bits than direct representation of the speech signal, so a reasonable LPC vocoder can operate at bits rates as low as 2-3 Kbps (kilobits per second) whereas the public telephone system uses 64 Kbps (8 bit PCM codewords at 8,000 samples per second). See for example, McCree et al, A 2.4 Kbit/s MELP Coder Candidate for the New U.S. Federal Standard, Proc. IEEE Int.Conf.ASSP 200 (1996) and U.S. Pat. No. 5,699,477.
However, the speech output from such LPC vocoders is not acceptable in many applications because it does not always sound like natural human speech, especially in the presence of background noise. And there is a demand for a speech vocoder with at least telephone quality speech at a bit rate of about 4 Kbps. Various approaches to improve quality include enhancing the estimation of the parameters of a mixed excitation linear prediction (MELP) system and more efficient quantization of them. See Yeldener et al, A Mixed Sinusoidally Excited Linear Prediction coder at 4 kb/s and Below, Proc. IEEE Int. Conf. Acoust., Speech, Signal Processing (1998) and Shlomot et al, Combined Harmonic and Waveform Coding of Speech at Low Bit Rates, IEEE . . . 585 (1998).